This tragedy was either the result of bad intel or mistaken identity. The BBC:
Afghan officials say all those killed were women and children.
The strike took place in Nawzad district after a US Marines base came under attack.
The air strike, targeted at insurgents, struck two civilian homes, killing two women and 12 children, reports say.
"The president called this incident a great mistake and the murdering of Afghanistan's children and women, and on behalf of the Afghan people gives his last warning to the US troops and US officials in this regard," his office said.
The White House said it shared Mr Karzai's concerns and took them "very seriously".
A group from Sera Cala village travelled to Helmand's capital, Lashkar Gah, bringing with them the bodies of eight dead children, some as young as two years old, says the BBC's Quentin Sommerville in Kabul.
"See, they aren't Taliban," they chanted as the carried the corpses to local journalists and the governor's mansion.
Before condemning NATO, perhaps we should ascertain the provenance of those dead bodies, as well as the political leanings of those who brought them to the governor's mansion. One of Saddam Hussein's stratagems was to parade the dead bodies of babies through the streets of Baghdad claiming they were victims of starvation caused by sanctions. In reality, they used the same dead children numerous times, fooling the western press into believing there was massive hunger in Iraq. There may have been people starving at that time, but it was not due to sanctions but rather Saddam's deliberately misdirecting oil for food funds into purchasing French and Russian weapons.
Any demonstration like this, planned and carried out for the benefit of the western press, demands more than passing scrutiny, and definitely should not be taken at face value. We are at war. And the idea that the enemy would not make such cynical use of the dead is naive.